The most prevalent destructive human disease is dental caries. Insidious enamel caries begins in the tooth surface and proceeds in a definite sequence to as yet irreversible subsurface demineralization. This incipient stage may arrest, but frequently leads to collapse and progression. The hypothesis is that caries is completely reversible in the enamel surface zone-onset stage. Insidious caries reversal with minerals resistant to later attack could lead to long-term prevention. The first aim is to characterize the chemical and structural changes of caries onset in the proven acid gelatin gel model that reproduces human-like enamel decay. The second goal is to quantitate the chemistry and structure of remineralization (reversal) of those onset-stage lesions and to test their resistance to further decay in the acid gelatin gel model. The third and long-term objective is to repeat aims above using a specific bacterial model for lesion development and enamel surface resistance to caries. Quantitative data will be obtained by calcium, phosphate, and fluoride analyses of demineralized and remineralized constituent; by iodide inhibition of lesions; by scanning electron microscopy; and by incident light microscopy coupled with computer analysis of affected enamel volumes. Long-term objectives are to derive and disseminate meaningful information on effective means of dental caries prevention. Knowledge of the onset-stage, surface limited lesions of enamel, and means of reversibility is sparse compared that of later stages of decay. Intact enamel surfaces will be subjected to 25% gelatin of 0.1 M lactic acid at pH 5.0 for short term intervals up to 3 days. The gel will be analyzed to determine rates of demineralization. Half lesions will be subjected to remineralization, and its extent estimated by loss of calcium, phosphate, and fluoride from solution and iodide inhibition. Microscopy will be used to sensitively document changes within the enamel. The methodology and results will provide a basis for understanding and communication between research groups that have in the past studied enamel caries with entirely different concepts and interpretations.